A. Field of the Invention
The invention disclosed herein relates generally to a multi-mesh gear system as well as to the design thereof. Generally, at least one presently preferred embodiment of the invention broadly relates to a multi-mesh gear system having design characteristics that reduce the self-excited component of dynamic load. It is also disclosed herein how to do so without significantly increasing the cost or complexity of manufacture. As a consequence of the reduced dynamic loading, the presently preferred multi-mesh gear systems disclosed herein generally have reduced noise and increased torque capacity or load-life performance when compared to conventional multi-mesh gear systems. Generally and broadly speaking, presently preferred gear systems and a presently preferred method of designing said gear systems that achieve desirable performance and cost characteristics are disclosed.
B. Description of the Prior Art
A multi-mesh gear system is characterized herein as a system having one or more gears, each of which is in mesh with two or more mating gears. Two gears are in mesh when the teeth of the two gears are in contact such that the gears will operate or turn together to turn other gears or other mechanisms such as drive shafts and similar machinery or components. Common examples of multi-mesh gear systems include epicyclic gear systems and idler gear systems. For the purposes of this disclosure, interchangeable stock gears can also be categorized as multi-mesh gear systems since the gears are intended to replace each other or other gears and mesh with many different mating gears.
An epicyclic gear system using spur gears will be used as the illustrative multi-mesh gear system for the purposes of this disclosure. However, this invention will apply equally to all multi-mesh gear systems and a variety of gear types. One skilled in the art will be able to extend the discussion herein to any multi-mesh gear system.
An epicyclic gear system is comprised of four elements: a sun gear, planet pinions, a planet carrier, and a ring gear. The sun gear meshes with at least one, but typically two or more planet pinions, also known as planet gears or planets. The planet pinions rotate about their own axes, revolve around the sun gear, and are mounted on a carrier, also known as the planet carrier. The planet pinions in turn also mesh with a ring gear, also known as an annulus gear. During rotation, a point on a planet pinion traces out epicyclic curves, hence the name.
As is characteristic of other multi-mesh gear systems, the planet pinions of an epicyclic gear system mesh with both the sun gear and the ring gear. Also, the planet pinion tooth flank in contact with the sun gear is opposite the planet pinion tooth flank in contact with the ring gear. The tooth flank is the portion of the planet pinion or any gear that comes in contact with another pinion or gear when the pinions or gears are in mesh. This is the characteristic that precludes full optimization of the complete multi-mesh system using conventional approaches for reducing noise and increasing torque capacity, as further detailed below.
It is well known in the art that as a pair of gears rotates, the effective stiffness of the gear mesh varies as a function of mesh cycle position. This periodic variation in mesh stiffness, referred to herein as mesh stiffness variation, gives rise to what has been referred to in the art as the dynamic increment of load. In addiction to serving as the primary excitation for gear noise, this dynamic increment serves to reduce the effective torque capacity of the gear mesh. A desirable design objective is to reduce the variation in mesh stiffness to the greatest degree possible. From a practical perspective, reducing mesh stiffness variation by a factor of two or more usually leads to an appropriate and desirable reduction in the dynamic increment of load, although any reduction in mesh stiffness variation will lead to a reduction in the dynamic increment of load that is desirable. Consequently, any means that can be employed to reduce this self-excited component of dynamic load will have the effect of reducing gear noise and increasing power density.
Conventional approaches to optimizing gear performance such as tip relief and root relief use initial separation to reduce mesh stiffness variation at one specific design load. At all other loads, the initial separation will be sub-optimal and in fact typically degrades performance. Further, the initial separation is optimized for the specific characteristics of one gear mesh between a first gear and a second gear and not for other meshes that exist between said first gear and other gears in the multi-mesh gear system. Thus, in multi-mesh gear systems, conventional approaches provide a solution that is optimized for only one load and only one mesh.
One approach for optimizing the performance of more than a single mesh in an epicyclic gear system is disclosed in U.S. Pat. No. 5,605,518 for a Planetary Gear Device wherein the tooth surfaces on one flank of the planet gears are optimized using tip and/or root relief for one mesh and the tooth surfaces on the other flank are optimized (also using tip and/or root relief) for the other mesh. This results in a multi-mesh system optimized for all meshes. There are, however, two major drawbacks to the disclosed solution. One, the manufacturing complexity and cost would significantly increase by requiring the manufacture of different profile modifications on the opposing tooth flanks of the planet gears. Two, the profile modifications specified in the referenced patent disclosure are limited to conventional modifications, such as tip and/or root relief, which utilize initial separation, and as a consequence, minimize mesh stiffness variation at one specific load only.
There is another means used in the industry aimed at reducing the mesh stiffness variation of epicyclic gear systems, known in the field as planet phasing. Several possible approaches to planet phasing are disclosed in U.S. Pat. No. 3,241,392 and further discussed in numerous published papers such as (a) J. Lin and R. G. Parker, xe2x80x9cStructured Vibration Characteristics of Planetary Gears with Unequally Spaced Planetsxe2x80x9d, Journal of Sound and Vibration (2000), (b) R. G. Parker, xe2x80x9cA Physical Explanation for the Effectiveness of Planet Phasing to Suppress Planetary Gear Vibrationxe2x80x9d, Journal of Sound and Vibration (2000), and (c) M. Gradu, K. Langenbeck, and B. Breunig, xe2x80x9cPlanetary Gears with Improved Vibrational Behaviour in Automatic Transmissionsxe2x80x9d, VDI Berichte NR1230, 1996.
Planet phasing makes use of equal or unequal planet pinion spacing to control the relative phase separation of each planet pinion mesh. While the mesh stiffness variation of each mesh is unchanged through the use of planet phasing, the stiffness variation of the system can be manipulated in a way that reduces system vibration, albeit in a manner that can be load specific and quite sensitive to gear and assembly related manufacturing inaccuracies. Embodiments of this invention can be used in conjunction with any of the various planet phasing strategies.
In contrast with the above previously disclosed methods, it would be desirable to achieve a fully optimized multi-mesh gear system across substantially all relevant operating loads. It would also be desirable to do so without significantly increasing manufacturing complexity or cost. Additional desirable performance advantages of such a multi-mesh gear system include reduced noise and increased torque capacity or load-life performance.
In general, in one presently preferred embodiment broadly in accordance with the invention, a multi-mesh gear system is provided that is optimized to minimize the self-excited component of dynamic load across substantially all operating loads without a significant increase in manufacturing complexity or cost. This should have the effect of reducing noise and improving load-life performance. Generally, in one preferred embodiment broadly contemplated by the invention, a multi-mesh gear system that utilizes substantially the same contact ratio for all meshes within the multi-mesh gear system in combination with the use of a mesh stiffness variation reducing (MSVR) type of gear tooth modification is disclosed herein.
An MSVR modification is defined herein as any system or form of gear tooth modification that makes substantially equal the mesh stiffness in the region where fewer teeth are in contact and the mesh stiffness in the region where more teeth are in contact by modifying the elastic tooth pair stiffness characteristic. An example of an MSVR modification is Differential Crowning, a system of gear tooth modifications disclosed in U.S. Pat. Nos. 5,485,761, 5,341,699, and 5,083,474.
Differential Crowning, for the purposes of this disclosure, can be defined as the introduction of different forms and amounts of lead crowning at different positions along the height of the gear tooth to reduce the variation in mesh stiffness that occurs as the number of tooth pairs in contact changes during a mesh cycle. Differential Crowning is effective at substantially all loads because it changes the tooth stiffness elastically rather than by introducing an initial separation of the teeth.
Another example of an MSVR modification is Variable Face Width Gearing, which is disclosed in U.S. Pat. No. 6,230,578.
All MSVR modifications utilize a change in gear tooth geometries at specific locations along the tooth height to achieve the desired mesh stiffness characteristic. The location of these geometry changes is directly related to the contact ratio. If contact ratio changes, the proper location of these gear tooth geometry modifications will also change. As a result, the design of the MSVR modification is different for gearsets having different contact ratios.
As an example, combining Differential Crowning with the Planetary Gear Device disclosed in (U.S. Pat. No. 5,605,518 would lead to an optimized system from a performance standpoint. However, due to the use of substantially different contact ratios in the planet-to-sun and planet-to-ring meshes, it is necessary to apply different forms and amounts of Differential Crowning on opposing tooth flanks, which would result in a complex and costly manufacturing process.
Contact ratio is a number that indicates the average number of pairs of teeth in contact during a full mesh cycle for a given gear mesh. For spur gears, it is defined as the length of the line of contact divided by the base pitch. In conventional designs, contact ratio is not usually a critical design parameter; rather, it is more typically a byproduct of other design parameters.
As a result, in conventional design of an epicyclic gear system, again being used as a representative example of a multi-mesh gear system, the contact ratio of the planet-to-sun gear mesh is typically different from the contact ratio of the planet-to-ring gear mesh. To optimize this type of system at a particular load, any type of profile modification, such as tip relief,has to be different fot the planet-to-sun mesh than it is for the planet-to-ring mesh. This means that either the modifications on the opposing flanks of the planet gears have to be different, or separate modifications need to be made to both the sun gear and the ring gear. As a result, manufacturing becomes complex and costly, and still results in a system that has been optimized for only one operating load.
Generally, in one presently preferred embodiment broadly contemplated by the invention, disclosed herein is a multi-mesh gear system that combines the use of an MSVR modification to the gear teeth with the purposeful use of a constant contact ratio throughout said system. This combination of features allows the same modifications to be specified for both planet tooth flanks, and results in a system that can be optimized for substantially all operating loads, rather than in a way that is load-specific. Any deviation from using exactly the same contact ratio throughout said system would have the effect of changing the harmonic signature of the noise characteristic, usually increasing sound power present at higher harmonics of the tooth contact frequency. Depending on the specific application and the desired noise characteristic, it is preferred that all of the contact ratios be maintained within 1-3% of each other, although, as broadly contemplated by the invention, contact ratios that achieve desirable noise or performance characteristics outside of this range are also desirable.
These and other advantages of the present inventions will be clarified in the detailed description of the preferred embodiments taken together with the attached figures wherein like reference numerals represent like elements throughout.